Electrical apparatus for destroying weeds is disclosed in such patents as U.S. Pat. No. 3,919,806 wherein a source of high voltage electricity is carried on a vehicle having a sharp-edge metallic ground contact wheel, or coulter, for connecting the low side of the high voltage source to the ground, and electrodes carried on the vehicle and connected to the high side of the voltage source are positioned to contact the weeds to transfer electrical current thereto to kill the weeds. The high voltage source may comprise an electrical generator driven by the power take-off shaft of the tractor pulling the vehicle and a transformer for stepping up the generator voltage. Grounding and safety features are provided to prevent the high voltage from harming the vehicle operator or bystanders. Specifically, U.S. Pat. No. 3,919,806 discloses four normally open interlock switches connected in series which must be closed in order to energize the transformer, and such interlock switches are controlled by respectively: (a) the transformer cover which must be closed; (b) the ground contact wheel which must be in position to penetrate the ground; (c) the "run" switch which is manually held by the operator and (d) means to indicate that the vehicle is travelling above a predetermined speed.
Such safety features substantially reduce hazards to the operator and bystanders, but the interlocks are not tamper-proof, can easily be disabled, and do not provide optimum reliability. Failure of a single interlock can result in a hazardous condition of the weed destroying apparatus. Also, such known weed killing apparatus does not have immediately-visible warning of the electrical hazard. If the coulter interlock switch is jammed closed, the high voltage source may be energized while the vehicle is at rest, and this presents a hazard to bystanders, and also to the vehicle operator since loss of ground contact can cause the vehicle or the tractor to assume the high potential at the output side of the transformer. Still further, even when all interlocks are operating satisfactorily and the vehicle is at rest, the residual generator voltage is applied to the primary side of the transformer and the voltage produced in the secondary is still high enough to create a hazard.